The present invention relates to biomedical electrodes. It finds particular application in electrodes for surrounding a nerve trunk to introduce electrical stimuli thereon for the purpose of generating action potentials that propagate in only one direction from the cuff. It is to be appreciated, however, that the present invention may find other utility.
Heretofore, others have used electrical stimuli to create action potentials that in turn cause the release of a neurotransmitter that may result in a measurable physiological response. Many of these prior art stimulation techniques applied the electrical stimuli to peripheral nerves subserving muscle or peripheral sense organs, bypassing the higher levels of the nervous system. It has also been found that electrical signals can be applied to other excitable tissue directly, such as muscle. Although the applied electrical signals can cause the nervous system to activate appropriate muscles or other organ responses, another potentially important use is to block the naturally generated action potentials traveling along a nerve fiber.
Specifically, the proper application of electrical signals can block nerve impulses traveling up the nerve trunk toward the brain to arrest pain signals, phantom limb signals in amputees, and the like. Analogously, appropriately generated action potentials can block nerve impulses traveling on the nerve trunk to eliminate nerve impulses which cause unwanted physiological activity. For examle, the stimuli can block signals which cause spasmodic behavior, hiccups, and the like. A potential application is to cause the controlled relaxation of the external urinary bladder or sphincter in paralyzed patients who have lost this control. With proper application of electrical signals, a paraplegic with a loss of voluntary bladder control can void the contents of the bladder.
Various electrical potentials have been applied between a cathode and an anode to suppress the transmission of unwanted nerve action potentials. Some researches have used DC currents flowing from the anode to the cathode to block natural nerve impulses. Others have used high frequency sinusoidal stimulation to block natural nerve impulses.
In the past, electrical stimuli have been applied to the nerves for the purpose of generating unidirectional propagating action potentials with cuffs containing three electrodes. The prior art electrode cuff included a dielectric, i.e., electrically non-conductive, cylindrical tube or sleeve. Three annular electrodes were disposed along the inner surface of the sleeve. That is, a cathode electrode was commonly positioned centrally in the sleeve and a pair of anode electrodes were positioned to either side thereof and displaced from the ends of the sleeve.
The electrodes have been utilized to introduce a string of artificially generated antidromic pulses which propagate unidirectionally in the opposite direction to the normal orthodromic pulse flow. The antidromic pulses collide with the natural orthodromic nerve impulses coming the other direction, blocking them from further propagation. An exemplary antidromic pulse generating electrical signal is a series of pulses each of which has a vertical leading edge, a period of constant amplitude followed by an exponentially decaying trailing edge.
Once inserted in the body, the three electrode cuff was immersed in electrically conductive body fluids and tissues. Electric currents were selectively caused to flow from both of the anodes by separate sources, through the conductive body fluids and tissues, through the nerve trunk to the centrally disposed cathode. The like diameter anodes and cathode were disposed along and within an insulating sleeve. In this manner, substantially all current flowed from the anodes to the cathode through the interior of the sleeve with little current flow around the exterior of the sleeve. The one anode at the escape end acts to suppress a secondary current path from the other anode, flowing out the insulating sleeve at the arrest end and entering the cuff from the escape end. This secondary current, which passed through the nerve, if not suppressed, can cause action potentials to be generated at a site away from the arrest end of the cuff by virtue of the direction of its flow. The current flowing from the anode, positioned at the escape end, is controlled by a current source that is separate from the one supplying the anode at the arrest end. The magnitude of the current flowing at the escape anode must be such that it will only suppress the secondary current flow arising from the arrrest end anode and not itself cause arrest of the approaching antidromic action potential.
The second current source would tend to create a number of problems with the prior art cuff electrodes. First, excess electrode current must be carried by the shared cathode. The excess current is that component applied for the purpose of suppressing the secondary current arising from the arrest end anode. This extra current could be injurious or cause the cathode to be unnecessarily large compared to the nerve. Further, the requirement for two separate current sources that are synchronously activated adds significant complexity to a cuff as opposed to using only a single current source.
The present invention contemplates a new and improved electrode cuff which eliminates (1) the second anode at the escape end, (2) the requirement for a second current source and (3) the high charge or current density at the cathode, yet overcomes the above-referenced problems and others.